docs: def RFC3339(self):
          """RFC3339.
  
          `Link to RFC3339.`__
  
          __ https://www.ietf.org/rfc/rfc3339.txt
          """
          # get timezone offset
          delta_sec = time.timezone
          m, s = divmod(delta_sec, 60)
          h, m = divmod(m, 60)
          # timestamp
          format_string = "%Y-%m-%dT%H:%M:%S.%f"
          out = self.datetime.strftime(format_string)
          # timezone
          if delta_sec == 0.:
              out += "Z"
          else:
              if delta_sec > 0:
                  sign = "+"
              elif delta_sec < 0:
                  sign = "-"
  
              def as_string(num):
                  return str(np.abs(int(num))).zfill(2)
  
              out += sign + as_string(h) + ":" + as_string(m)
          return out